Since standards relative to air pollution become more and more severe in most industrialized countries, car manufacturers carry out extensive research, in various ways, in order to improve the efficiency of catalytic mufflers.
One major problem posed by catalytic mufflers relates to cold starting: when the temperature of the exhaust gases does not reach a given threshold value, the catalytic muffler actually has no effect on the pollutants contained in the exhaust gases. The starting period of a catalytic muffler is the time during which it has no or very little effect on pollutants. This period may usually last for several minutes.
Several solutions have been proposed to reduce the starting period of catalysts.
It is for example well-known, through patent applications EP-0,177,479; U.S. Pat. No. 4,376,374; EP-0,340,946 . . . , to carefully insulate the catalyst and/or the exhaust manifold in order to limit heat losses and therefore to allow the catalyst to heat up more rapidly. Overheating of the catalysts, when started, may be a drawback of such systems.
Another solution consists of creating different flows for the exhaust gases, according to their temperature. Patent FR-2,608,678 and document DE-3,406,968 disclose solutions of this type which, however, cannot always be easily implemented.
Another way of improving the efficiency of a catalytic muffler on the cold starting of the vehicle consists of bringing the muffler closer to the engine. But this is not always possible because of the size of the muffler and of the space available in the vehicle. One therefore often has to use several mufflers in series: a first muffler of limited size which is placed as close as possible to the engine, followed by a second muffler of larger size placed further from the engine. The first muffler, which is used to speed up depollution upon starting, is commonly called "light-off muffler".
Multicylinder engines having tuned exhaust manifolds, designed to improve the filling of the engine and its power according to the pressure waves in the exhaust at the various engine speeds, may have several "light-off mufflers", each being arranged in the exhaust manifold of a cylinder or of a given group of cylinders.
With two-stroke engines, the power of the engine is substantially linked to the geometry of the exhaust line upstream from the gas expansion vessel, so that the presence of a light-off catalyst, even of small size, may considerably harm the running of the engine through disturbances of the pressure waves in the exhaust.
This is the reason why no conventional light-off catalysts can be used in exhaust lines of two-stroke engines.
On the other hand, it has been proposed to replace light-off mufflers by a catalytic coating applied directly on the inner walls of exhaust manifolds. Document SAE-790,306, published in March 1979, and entitled "Catalyst systems with an emphasis on three-way conversion and novel concepts", contemplates such a coating. According to this publication, the catalyst is applied either directly on the inner wall of the exhaust tube, or on metallic crossbraces welded onto the inner wall of the exhaust tube.
Although this type of catalyst is advantageous as far as pressure waves are concerned, the problem of the heating period of the catalyst remains since the wall of the tube, cooled by the outer air, takes a long time to heat up.